(1) Field of the Invention
This invention relates generally to image processing and relates more particularly to a method to reduce the fixed pattern noise in an obtained image.
(2) Description of the Prior Art
CMOS-based cameras are being plagued by a defect known as “fixed pattern noise” (FPN) This is an annoying stationary background pattern in the image that results from small differences in the behaviour of the individual pixel amplifiers. FPN (also called nonuniformity) is the spatial variation in pixel output values under uniform illumination due to device and interconnect parameter variations (mismatches) across the sensor. FPN is fixed for a given sensor, but varies from sensor to sensor, so if vo is the nominal pixel output value (at uniform illumination), and the output pixel values (excluding temporal noise) from the sensor are voij for 1≦i≦n and 1≦j≦m, then the fixed pattern noise is the set of values Δvoij=voij−vo.
FPN consists of offset and gain components, the offset component causes more degradation in image quality at low illumination. CMOS (Active Pixel Sensors—APS— and Passive Pixel Sensors—PPS—) suffer from column FPN, which appears as “stripes” in the image and can result in significant image degradation. CCD image sensors only suffer from pixel FPN due to spatial variation in photodetector geometry and dark current. Using CMOS technology image sensors pixel transistors cause additional pixel FPN and column amplifiers cause column FPN.
In prior art, the normal way to reduce FPN is to do a so-called Correlated Double Sampling (CDS). This involves sampling the output twice, once right after reset and the second time with the signal present. The offset and reset noises are reduced by taking the difference. Most sensors cannot implement a true CDS and thus cannot fully compensate offset FPN.
Dark frame subtraction comprises to capture one or more dark frames under the actual picture taking conditions (exposure time and sensor temperature), and the frame or average of multiple frames is subtracted from the image. Dark frame compensation can be used but requires an external shutter and a full frame memory and is therefore an expensive solution to reduce the FPN problem.
There are patents dealing with the suppression of FPN:
U.S. Pat. No. 6,061,092 to Bakhle et al. describes a method and an apparatus for dark frame cancellation for CMOS sensor-based tethered video peripherals. The elimination of dark fixed pattern noise (DFPN) for tethered CMOS sensor-based digital video cameras is supported by supplying and maintaining a host-based dark image cache. Since the camera is tethered to a host computer system such as a PC, it takes advantage of the storage and processing capabilities of the host to manage the cache. By using a dark image cache for updating of the currently applicable dark image for DFPN cancellation processing, operation of the camera shutter for acquiring dark images is dramatically reduced. Dark images are obtained at different integration, gain, and temperature operating characteristics of the camera and stored in the cache. The cached dark images are referenced on the host according to a fixed, predetermined column of data in video frames generated by the CMOS sensor image array of the camera. The dark column data represents a portion of the CMOS sensor image array that is permanently and totally shadowed for use during DFPN cancellation processing.
U.S. Pat. No. 6,215,113 to Chen et al. discloses a CMOS active pixel sensor with motion detection including a photo diode, a reset switch, two sample and hold circuits, and two readout circuits. In addition to a CMOS sensor array, a row of CMOS active pixel sensors (APS) can be added to reduce the fixed pattern noise caused by process variations. Each of the CMOS APS cells of this additional row is the same as the CMOS APS cells in said CMOS sensor array except that the photo diode area thereof is covered by a metal layer to block the incident light. Therefore, the readout signals of the pixels in this additional row should be fixed signals after the reset process and should be independent of the incident light. When an image signal of a selected APS cell is read out, a signal of the shielded APS cell in the same column with the selected APS cell is also read out. The final image is the difference of these two signals. Since these two readout values pass through the same readout circuit, the effect of process variation can be cancelled and the fixed pattern noise can be reduced.
U.S. Pat. No. 6,366,320 to Nair et al. discloses a semiconductor circuit having an analog storage array and a sense amplifier array in which each sense amp cell generates a differential signal pair in response to receiving first and second signals from the storage array. The sense amp array is a row of sense amplifier cells, one sense amp cell for each column. The sense amplifier cells implement correlated double sampling (CDS) of pixel signals. CDS is used to reduce fixed pattern noise in pixel signals. Fixed pattern noise refers to errors in otherwise identical pixel signal values that are caused by manufacturing variations between the pixels. CDS requires sampling both the pre-exposure “reset value” and the post-exposure “exposed value” of each pixel signal. The two values are then transferred as a differential signal pair to a signal-processing pipe. There the values are correlated or simply subtracted to cancel the errors caused by differences between reset values and circuitry in the pixels of the manufactured array.